High frequency electric cables that take up as little room as possible, i.e. that are capable of withstanding considerable bending stresses and thus considerable curvature, are being sought after more and more at present in order to save space, in particular in aviation, military, and space applications. Such very flexible cables are also required to have mechanical endurance (i.e. good resistance to periodically repeated stresses) and electrical performance that is acceptable given the applications concerned.
In such cables, flexibility problems generally arise with their inner conductors.
In particular, two types of coaxial cable are known at present which satisfy either requirements in terms of flexibility, or else requirements in terms of linear attenuation.
For example, a first type of coaxial cable having low linear attenuation comprises the following disposed coaxially from the inside towards the outside:
a central core constituted by a solid metal conductor called the solid core;
a covering of dielectric material, generally having a relative density that is greater than one;
an outer conductor constituted, for example, by a braid of metal tapes having a braid of circular section wires superposed thereon; and
an outer protective sheath of insulating material.
A cable of that type is considered as being satisfactory from the point of view of linear attenuation: at 1 GHz this is generally about 0.12 dB/m to 0.13 dB/m for a cable having a diameter of 10 mm.
In contrast, such a cable has a minimum radius of curvature that is equal to about eight times its outside diameter, and its mechanical endurance is poor. For radii of curvature smaller than the above value, the solid core of the cable is subjected to harmful degradation. Indeed, it is because the material constituting the covering has a relative density greater than one that the central core is supported mechanically and the above-mentioned linear attenuation values are guaranteed so long as the radius of curvature imparted to the cable is equal to eight times its outside diameter.
To increase the flexibility of such cables, proposals have been made to replace the solid central core with twisted-together conductive wires constituting a "divided core" for the cable, and the dielectric material constituting the covering is constituted by a material whose relative density is generally less than one.
Under such circumstances, the minimum radii of curvature that can be achieved are about four to five times the outside diameter of the cable, which constitutes a considerable improvement over the above solid core cables, and mechanical endurance is improved.
Unfortunately, the electrical performance of such cables is not very satisfactory compared with that of solid core cables. In particular, a divided core cable whose central conductor has a core diameter equal to the diameter of the solid core of the corresponding solid core cable (where the term "core diameter" designates the diameter of the circle circumscribing the twisted wires) suffers from linear attenuation that is about 30% greater than that of the solid core cable.
Similar problems are observed in symmetrical pair cables where two insulated internal conductors which may be solid or divided are included within a protective sheath.
Thus, the internal conductors used in various known high frequency electric cables are not capable of simultaneously satisfying requirements in terms of electrical performance (linear attenuation at 1 GHz close to about 0.12 dB/m to 0.13 dB/m for a cable having a diameter of 10 mm), and to mechanical endurance and flexibility requirements (minimum radius of curvature about three to five times the outside diameter of the cable).